The present invention relates to both a method and a control system for adjusting the load exerted by a draft implement such as a plow or a cultivator.
It is known to adjust the depth of a draft implement in inverse relation to a measurement of draft load. One of two reasons are typically given for this adjustment. One reason is that without depth adjustment the vehicle engine may stall or incur damage when an excess draft load is encountered such as when encountering heavy soil conditions during field plowing. Another reason is that without depth adjustment, the depth of the draft implement increases when the towing vehicle encounters an incline and, conversely, decreases when the towing vehicle encounters a decline. By adjusting the depth of the draft implement in inverse relation to an indication of draft load, a more uniform plowed field is obtained when a field grade is encountered.
The measurement of draft load has commonly been provided by coupling a force transducer to one of the linking arms attached to the draft implement. However, force transducers must operate in an exposed, hostile environment. Further, since the actual draft force is distributed over several linking arms, an accurate measurement of draft force is not obtained from a single link arm.
Various approaches have therefore evolved wherein the measurement or indication of draft load is inferred from a measurement of engine parameters. In one approach, draft load is inferred from a deviation in measured engine rpm from a single set value of engine rpm associated with maximum horsepower output. A problem with this approach is that the system is prone to fluctuate or search around a desired depth associated with the set value of engine rpm. Thus, even without variations in soil density, or variations in grade, the plowed field depth will fluctuate.
In another approach, as disclosed in U.S. Pat. No. 4,077,475, draft depth is varied in inverse relation to a combination of rpm deviation and actual implement depth. By adding feedback for actual implement depth, the effect of depth variation with variations in engine rpm is allegedly dampened. The inventors herein have recognized, however, that the degree of dampening is not adjustable to compensate for a change in the draft implement used or for changes in field soil conditions.
A disadvantage with the above approaches is that draft vehicles are usually operated near the peak horsepower of the engine wherein the torque versus rpm curve is relatively flat. Substantially large variations in engine rpm are therefore associated with relatively small variations in draft load. Accordingly, inferring load by an rpm deviation from a desired rpm value is not satisfactory.